The present invention relates to a control device for a battery charging AC generator used in a motor vehicle, and, in particular, relates to a semiconductor control device for controlling the output voltage of a battery charging AC generator driven by an internal combustion engine via a belt in a motor vehicle.
Motor vehicles using an internal combustion engine as their propelling power source, such as automobiles, require many kinds of electrical devices. Among the required electrical devices some of them such as a starter are required to be operated even when the internal combustion engine is in a stand-still condition.
Therefore, it is a common practice in such motor vehicles to provide a battery and to charge the battery using a generator driven by the internal combustion engine in order to supply electric power to the respective electrical devices. For this reason, in such motor vehicles, a battery charging control device is used which controls the output voltage of the generator so that the battery is always kept in a proper charging condition.
In such a battery charging control device, a generating voltage control at a proper level is performed by controlling the current, particularly the field current, flowing through the field winding of the generator for the motor vehicle.
JP-A-1-283030(1989) discloses one example of conventional semiconductor battery charging control devices in which a semiconductor power switching element is used and the field current is controlled by means of pulse width modulation (PWM) through on-off control of the semiconductor power switching element.
JP-A-63-18933(1988), which corresponds to U.S. Pat. No. 4,754,212, discloses another example of conventional semiconductor battery charging control devices in which a generated output voltage is regulated by making use of an output condition of a semiconductor power switching element as indicated by a flip-flop circuit.
JP-A-62-64299(1987), which corresponds to U.S. Pat. No. 4,636,706, and JP-A-2-184300(1990), which corresponds to U.S. Pat. No. 5,140,253, disclose further examples of conventional semiconductor battery charging control devices in which a similar flip-flop circuit is employed and a gate signal for a semiconductor power switching element is latched by a timing pulse.
In the above mentioned conventional semiconductor battery charging control devices, no measure was taken against the undesired possibility that the semiconductor power switching element may undergo additional on-off operation due to noise and disturbances caused by, for example, a protection circuit, or the possibility that the frequency of the actual on-off operation, i.e. the switching frequency, of the semiconductor switching element will exceed the PWM frequency, i.e. the frequency of carrier wave for the PWM as a result, certain problems have arisen in conventional device, such as an increase of switching loss and increase of electromagnetic induction noise due to the high speed switching.
With the above explained conventional semiconductor battery charging control device in which the gate signal is latched in response to a timing pulse by making use of a flip-flop circuit, the on-off frequency fluctuation is comparatively suppressed, however because of the requirements of the timing pulse generating circuit, the circuit scale of the semiconductor battery charging control device is increased which causes an increase in the cost of the device.